This invention relates to an oxygen sensor for detecting a partial pressure of oxygen in the surrounding atmosphere.
In combustion apparatus (e.g., an internal combustion engine), an oxygen partial pressure for the exhaust gas is detected by an oxygen sensor, and the air/fuel ratio of the gas mixture combusted in the apparatus is adjusted to a desired value. The fuel consumption rate and the emission are thus improved to attain the optimum driving conditions.
An example of general oxygen sensors (Japanese Patent Application No. S61-146210) is shown in the partially sectional view of FIG. 2. The oxygen sensor of FIG. 2 includes a detection element 16 consisting of a test-tube shaped oxygen-ion-conducting solid electrolyte 10 (e.g., ZrO.sub.2) and a pair of porous electrodes 12 and 14 on each face of the electrolyte 10. The oxygen sensor has: a metal member 18; the detection element 16 mounted in the member 18 with an insulating ceramic spacer 20, packed talc powders 22, and a ceramic sleeve 24; a protection tube 26 for the detection element 16; a metal terminal 28 connected to the external electrode 12 of the detection element 16; a metal terminal 32 connected to the inner electrode 14 of the detection element 16; a metal terminal 33 of an internal ceramic heater 30; leads 34, 36, 38, and 39 respectively connected to metal terminals 28, 32, and 33, and the ceramic heater 30; and housings 40 and 42 for protecting the terminals 28, 32, and 33.
In this oxygen sensor, an oxygen partial pressure signal generated between the two porous electrodes 12 and 14 of the detection element 16 is outputted from the leads 34 and 36 via the metal terminal 28 for the external electrode 12 and the metal terminal 32 for the inner electrode 14 respectively. The oxygen sensor can thus accurately generate the oxygen partial pressure signal without being influenced by noise from an ignition plug or from the ground voltage, compared to another oxygen sensor in which an external porous electrode of a detection element is grounded to the exhaust system via a metal member. An example of the latter type of oxygen sensor is Japanese Patent Publication No. S59-41952.
The above oxygen sensor, however, has the following problem. When a fuel excess period temporarily occurs owing to an application of an accelerator, the carbon particles that are produced adhere to and accumulate on a surface S of the insulating spacer 20 and surfaces of other components, thus causing the deterioration of the insulation between the porous electrode 12 of the detection element 16 and the metal member 18. When the insulation deteriorates, the oxygen partial pressure signal might not be accurately detected due to the noise from an ignition plug or a potential difference from the ground. The carbon particles accumulate on the above site because the temperature is low at the metal member 18 side of the detection element 16. The temperature is comparatively high at the tip of the detection element 16, thereby allowing the carbon particles to be burned off.